Gas separation by pressure swing adsorption (PSA) and other adsorptive gas separation processes such as temperature swing adsorption (TSA) and partial pressure swing or displacement purge adsorption (PPSA) are achieved when a first gas component is more readily adsorbed on an adsorbent material compared to a second gas component which is relatively less readily adsorbed on the adsorbent material. In many important applications, to be described as “equilibrium-controlled” processes, the adsorptive selectivity is primarily based upon differential equilibrium uptake of the first and second components. In another important class of applications, to be described as “kinetic-controlled” processes, the adsorptive selectivity is primarily based upon the differential rates of uptake of the first and second components.
In PSA processes, a feed gas mixture containing the first and second gas components is separated by cyclic variations of pressure coordinated with cyclic reversals of flow direction in a flow path contacting a fixed bed of the adsorbent material in an adsorber vessel. In the case of TSA or PPSA processes, cyclic variations of temperature and/or partial pressure of the gas components may be coordinated with gas flow through a flow path to perform a separation. The process in any specific PSA application operates at a cyclic frequency characterized by its period, and over a pressure envelope between a first relatively higher pressure and a second relatively lower pressure. Separation in PSA is achieved by coordinating the pressure variations with the flow pattern within the flow path, so that the gas mixture in the flow path is enriched in the second component (owing to preferential adsorptive uptake of the first component in the adsorbent material) when flowing in a first direction in the flow path, while the gas mixture is enriched in the first component (which has been desorbed by the adsorbent material) when flowing in the opposite direction in the flow path. In order to achieve separation performance objectives (i.e. product gas purity, recovery and productivity), process parameters and operating conditions should be designed to achieve a sufficiently high adsorptive selectivity of the first and second components over the adsorbent material, at the cyclic frequency and within the pressure envelope.
In PSA processes designed to be equilibrium-controlled, the intrinsic adsorptive selectivity may typically be independent of cycle frequency, and depend only on the intrinsic equilibrium adsorptive preference of the adsorbent material in question relative to the fluid components in the feed fluid. The actual separation performance may be degraded by dissipative effects including mass transfer resistance and axial dispersion. The deleterious effects of mass transfer resistances associated with film (such as resulting from fluid flow boundary layer effects), macropore and micropore mass transport on equilibrium-controlled separation performance typically increase at higher gas flow velocities associated with higher cycle frequencies. Therefore the maximum practicable PSA cycle frequency which can be achieved for equilibrium-controlled separations may typically be limited by such mass transfer resistances. In order to maximize specific productivity for a given adsorbent vessel volume, it is desirable to increase cycle frequency within the constraints set by (1) performance degradation associated with mass transfer resistance and (2) adsorbent degradation as fluidization velocities of typical conventional granular packed beds are approached. While smaller adsorbent pellets may typically have lower macropore and film mass transfer resistance, it is generally impracticable to reduce pellet diameters below about 0.5 mm to about 1 mm before encountering excessive flow friction pressure gradients and the risk of fluidization and associated adsorbent pellet degradation.
As set forth in prior commonly assigned U.S. Pat. Nos. 5,082,473, 6,451,095, 6,692,626, and U.S. patent application Ser. No. 10/041,536 (the contents of which are herein incorporated by reference), equilibrium-controlled PSA processes may be enhanced by configuring the adsorbers as layered “adsorbent laminate sheet” parallel passage contactor structures, with the adsorbent material formed into adsorbent sheets, with or without suitable reinforcement materials incorporated into such sheets. As described in the above references, such adsorbent sheets may preferably be separated by spacing means, such as exemplary expanded or woven metal mesh sheet spacers, or printed spacers, establishing generally parallel fluid flow channels between adjacent surfaces of adsorbent sheets. Such parallel passage adsorbent contactor structures may be assembled according to methods known in the art, such as by the exemplary forming of the adsorbent sheets and spacing means as stacked layers or as a multi-layer spiral roll. While parallel passage adsorbers fabricated as extrudate monoliths are also known in the art, the adsorbent structures formed from multiple layers of adsorbent sheets, as described above are particularly suitable for achieving high surface area and narrow flow channels desirable for cyclic adsorptive service.
It has been established that multilayer adsorbent sheet structures can achieve favourable performance in equilibrium-controlled PSA processes where macropore diffusion dominates mass transfer resistance. The adsorbent is fully immobilized in sheet form to avoid fluidization limits of conventional granular adsorbers. Flow friction pressure drop is reduced relative to conventional packed granular beds, while macropore mass transfer resistance may be reduced by using thin adsorbent sheets. Because mass transfer and pressure drop constraints can be reduced, equilibrium-controlled PSA processes can thus be operated at high cycle frequency, for example and without limitation, up to about one cycle per second for adsorbent sheets about 250 microns thick. Therefore, compared to conventional granular (beaded) adsorbent beds, the onset of separation performance degradation due to macropore mass transfer resistance in adsorbent sheet structures can be shifted to higher operating cycle frequencies (as determined by macropore kinetics), while the inherent selectivity of the equilibrium-controlled process remains substantially unaffected.
In kinetic-controlled adsorption processes, separation over a given adsorbent material may be achieved between a first component which adsorbs and typically also desorbs relatively more rapidly at a particular cycle frequency, and a second component which adsorbs and typically desorbs relatively less rapidly at the cycle frequency. In the case of kinetic-controlled PSA processes, such adsorption and desorption are typically caused by cyclic pressure variation, whereas in the case of TSA, PPSA and hybrid processes, adsorption and desorption may be caused by cyclic variations in temperature, partial pressure, or combinations of pressure, temperature and partial pressure, respectively.
In the exemplary case of PSA, kinetic-controlled selectivity may be determined primarily by micropore mass transfer resistance (e.g. diffusion within adsorbent particles or crystals) and/or by surface resistance (e.g. narrowed micropore entrances). For successful operation of the process, a relatively and usefully large working uptake (e.g. the amount adsorbed and desorbed during each cycle) of the first component and a relatively small working uptake of the second component may preferably be achieved. Hence, the kinetic-controlled PSA process may preferably be operated at a suitable cyclic frequency, balancing between and avoiding excessively high frequencies where the first component cannot achieve a useful working uptake, and excessively low frequencies where both components approach equilibrium adsorption values.
Some established kinetic-controlled PSA processes use carbon molecular sieve adsorbents, e.g. for air separation with oxygen comprising the first more-adsorbed component and nitrogen the second less adsorbed component. Another example of known kinetic-controlled PSA is the separation of nitrogen as the first component from methane as the second component, which may be performed over carbon molecular sieve adsorbents or more recently as a hybrid kinetic/equilibrium PSA separation (principally kinetically based, but requiring thermal regeneration periodically due to partial equilibrium adsorption of methane on the adsorbent material) over titanosilicate based adsorbents such as ETS-4 (such as is disclosed in U.S. Pat. Nos. 6,197,092 and 6,315,817 to Kuznicki et. al.). These known kinetic-controlled adsorptive separation applications may be characterized by relatively low cycle frequencies and gas flow velocities, where conventional beaded or extruded adsorbents may be used in granular packed beds without resulting in fluidization. Since the cycle frequency of kinetic-controlled PSA processes is typically determined by micropore and/or surface resistance kinetics, contrasting with equilibrium-controlled PSA processes whose cycle frequency is typically limited by macropore kinetics, a system has been needed which can enable significant intensification (e.g. higher operating frequencies and gas flow velocities, and therefore resulting higher productivities and/or recoveries) of kinetic-controlled adsorption processes, such as PSA, TSA, and PPSA processes, and combinations thereof.